It is a general tendency that the performance, availability, and reliability of networks based on the internet protocol, so-called internet protocol (IP) networks is increasing. Similarly, switched circuit networks (SCN), including public switched telephone networks (PSTN), have been developed and improved for many decades and switched circuit networks work very reliably. Switched circuit networks are commonly used to interconnect mobile and fixed telephones. As the number of users and their respective bandwidth requirements increase it becomes necessary to increase the capability of switched circuit networks. Expanding and maintaining switched circuit networks requires however massive investments which could be saved to some extend if telephone services could be provided by IP networks. Switched circuit network carriers are therefore willing to consolidate both, IP networks and switched circuit networks.
In a switched circuit network two types of data traffic are used: media data traffic and signaling data traffic. Both types of data traffic can be migrated separately from switched circuit networks to IP networks, because they rely on different technologies.
In most public switched circuit networks the signaling data traffic is carried in a packet network which is referred to as Signaling System 7 (SS7). The Internet Engineering Task Force (IETF) has initiated the SIGTRAN working group which defined open standards for transporting SS7 signaling data traffic over IP networks. The architecture that has been defined by the SIGTRAN working group consists of three components: a standard internet protocol, a common signaling transport protocol that supports a common set of reliable transport functions for signaling transport which is referred to as stream control transport protocol (SCTP), and an adaptation sub-layer that supports specific primitives, such as management indications that are required by a particular signaling application protocol.
One new adaptation sub-layer amongst others is the Signaling Connection Control Part (SCCP) user adaptation layer which is abbreviated as SUA layer. On the SUA layer a protocol is defined for the transport of any SCCP user signaling. This protocol is called SUA protocol and defined by IETF.
For this protocol the IETF has defined two main entities, a signaling gateway and a SUA application server. The signaling gateway interconnects a SS7 network and an IP network. One or more SUA application servers are located on the IP network which can be requested by a SCCP user signaling on the SS7 network via the signaling gateway. Alternatively, a SUA application server on the IP network can request a SCCP user signaling on the SS7 network via the signaling gateway. A SCCP user signaling is also called a SCCP service and a SUA application server is also referred to as application server.
An SCCP service in the SS7 network is declared by use of a point code (PC) and a subsystem number (SSN). On the SUA layer, an application server is declared by a point code, a subsystem number and other parameters such as for example a global title (GT), a calling party address, or a mobile application part (MAP) parameter. Several application servers can share a point code and a subsystem number. They are separated by an additional parameter such as the MAP parameter which is unique to a specific application server comprised in the group of application servers that share the same point code and subsystem number.
When the signaling gateway receives a service call from the SS7 network which requests a service, the service call is routed to the application server which implements the service on the internet protocol network if the application server is available. If the application server is unavailable, the signaling gateway informs the calling party which has sent the service call by use of a control message that the application server is unavailable. The situation becomes more complex if a service call is routed to several application servers of which some are available and others are unavailable or if the service call is routed to an application server which is not available but the application server shares its point code and its subsystem number with another application server. For such complex situations it is not clear when control messages are sent to a calling party. There is therefore the need for an improved method for the distribution of control messages to calling parties.